lm5045sqx National Semiconductor Corporation, lm5045sqx Datasheet - Page 25

• Manufacturer Part Number: lm5045sqx
• Description: Full-bridge Pwm Controller With Integrated Mosfet Drivers
• Manufacturer: National Semiconductor Corporation

HICCUP MODE CURRENT LIMIT RESTART
The operation of the hiccup mode restart circuit is explained
in the Functional Description section. During a continuous
current limit condition, the RES pin is charged with 30µA cur-
rent source. The restart delay time required to reach the 1.0V
threshold is given by:
This establishes the time allowed before the IC initiates a hic-
cup restart sequence. For example, if the C
time TCS as noted in Figure 20 below is 334µs. Once the RES
pin reaches 1.0V, the 30µA current source is turned-off and
a 10µA current source is turned-on during the ramp up to 4V
and a 5µA is turned on during the ramp down to 2V. The hic-
cup mode off-time is given by:
With a C
cup time is finished, the RES pin is pulled-low and the SS pin
is released allowing a soft-start sequence to commence.
Once the SS pin reaches 1V, the PWM pulses will commence.
The hiccup mode provides a cool-down period for the power
converter in the event of a sustained overload condition there-
by lowering the average input current and temperature of the
power components during such an event.
PRINTED CIRCUIT BOARD LAYOUT
The LM5045 current sense and PWM comparators are very
fast and respond to short duration noise pulses. The compo-
nents at the CS, COMP, SLOPE, RAMP, SS, SSSR, RES,
UVLO, OVP, RD1, RD2, and RT pins should be physically
RES
=0.01µF, the hiccup time is 49ms. Once the hic-
FIGURE 20. Hiccup Mode Delay and Soft-Start Timing Diagram
RES
=0.01µF, the
25
close as possible to the IC, thereby minimizing noise pickup
on the PC board trace inductance. Eliminating or minimizing
via’s in these critical connections are essential. Layout con-
sideration is critical for the current sense filter. If a current
sense transformer is used, both leads of the transformer sec-
ondary should be routed to the sense filter components and
to the IC pins. The ground side of the transformer should be
connected via a dedicated PC board trace to the AGND pin,
rather than through the ground plane. If the current sense cir-
cuit employs a sense resistor in the drive transistor source,
low inductance resistors should be used. In this case, all the
noise sensitive, low-current ground trace should be connect-
ed in common near the IC, and then a single connection made
to the power ground (sense resistor ground point).
The gate drive outputs of the LM5045 should have short, di-
rect paths to the power MOSFETs in order to minimize in-
ductance in the PC board. The boot-strap capacitors required
for the high side gate drivers should be located very close to
the IC and connected directly to the BST and HS pins. The
VCC and REF capacitors should also be placed close to their
respective pins with short trace inductance. Low ESR and
ESL ceramic capacitors are recommended for the boot-strap,
VCC and the REF capacitors. The two ground pins (AGND,
PGND) must be connected together directly underneath the
IC with a short, direct connection, to avoid jitter due to relative
ground bounce.
Application Circuit Example
The following schematic shows an example of a 100W full-
bridge converter controlled by LM5045. The operating input
voltage range is 36V to 75V, and the output voltage is 3.3V.
The output current capability is 30 Amps. The converter is
configured for current mode control with external slope com-
pensation. An auxiliary winding is used to raise the VCC
voltage to reduce the controller power dissipation.
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